1. Field of the Invention
The present invention relates generally to a stereomicroscope having a transmission illumination apparatus, and more particularly to a stereomicroscope capable of interchangeably fitting a high magnification objective lens and a low-magnification objective lens as an objective lens.
2. Related Background Art
What is known as a conventional transmission illumination apparatus of a stereomicroscope is an apparatus disclosed in Japanese Utility Model Post-Exam Publication No. 41-5808. This transmission illumination apparatus is, as shown in FIG. 10 constructed such that divergent beam of light emitted from a light source 101 are collimated by a collector lens 102 into the light beam substantially parallel to an optical axis 108, and the collimated light beam is given a diffusion by a diffusion plate 103. A light path of the light beam is deflected upwards by a mirror 104 and converged by a condenser lens 105 so that the light beam may impinge upon a specimen 106 and illuminate the specimen 106. At this time, the diffusion plate 103 serves as a secondary light source to illuminate a pupil of the objective lens with the light beam. Further, a knife edge 107 is slidably disposed in the vicinity of the diffusion plate 103. Hence, the pupil of the objective lens is partially shielded from the light by sliding the knife edge, whereby the specimen 106 undergoes an oblique illumination. Even when the specimen 106 is a phase object, a ratio of diffracted light to the transmitted light can be thereby increased, and the specimen can be observed with a high contrast. Further, the light is not allowed to enter directly the objective lens by increasing a degree of shielding, thereby making it feasible to attain a dark field illumination with which only the light diffracted or scattered by the specimen 106 is observed.
Moreover, another conventional transmission illumination apparatus is disclosed in Japanese Patent Application Laid-Open Publication No. 11-133308. In this transmission illumination apparatus, as shown in FIG. 11, the light beam from a light source 111 is collimated by a collector lens 112 into substantially parallel light beam, and a first diffusion plate 113 diffuses the light beam. The light beam is converged by a converging member 114 and, after being diffused by a second diffusion plate 115, is deflected upwards by a deflection mirror 116. The light beam from the deflection mirror 116 is converged by a second converging member 117 and impinges upon a specimen 118, thus illuminating the specimen 118. Light shield members 121c, 121d for an oblique illumination are disposed between the second diffusion plate 115 and the deflection mirror 116. Further, when the objective lens is switched over from a low-magnification lens to a high-magnification lens, a conjugate position of a pupil of the objective lens deviates, and hence the light shield members 121c, 121d are removed, and light shield members 121a, 121b are inserted. Moreover, auxiliary lenses 119, 120 for intensifying the light convergence are also inserted for a high-magnification objective lens having a larger numerical aperture.
An application and a using purpose of the stereomicroscope have been diversified such as being used for inspection in a parts shop etc and for an embryo manipulation of a living body in a gene laboratory etc, and hence a variety of performances have been demanded of the stereomicroscope. A trend over the recent years has, however, been that a stereomicroscope capable of interchangeably fitting a plurality of objective lenses for broadening a magnification range and of having a wide visual field and a high resolution.
If trying to broadening an illumination range for securing a wide visual field in the conventional transmission illumination apparatus disclosed in Japanese Utility Model Post-Exam Publication No. 41-5808, it is required that a degree of diffusion of the diffusion plate 103 be increased. If the degree of diffusion is increased, however, a light intensity per unit area is reduced, and therefore the illumination darkens. Further, if a diameter of the pupil is increased for conducting an illumination corresponding to a numerical aperture of the objective lens having the high resolution in order to actualize the high resolution, there is no alternative but to scale up the diffusion plate 103, resulting in an increase in thickness or size of the illumination apparatus. Consequently, the stereomicroscope for the manipulation comes to have a poor usability.
Further, the transmission illumination apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 11-133308 is, though capable of interchangeably fitting the high- and low magnification objective lenses, required to remove and insert the light shield members disposed in two positions with respect to the optical axis, corresponding to a deviation in conjugate position of the pupil of the objective lens depending on which lens, the high-magnification objective lens or the low-magnification objective lens, is used. Another requirement is that the auxiliary lenses as many as two units be simultaneously inserted. Therefore, the number of components increases, and the apparatus becomes complicated, resulting in an increase in cost.
It is primary object of the present invention to provide a stereomicroscope capable of observing an object by a stable oblique illumination with a simple configuration in a broad magnification range from a high magnification down to a low magnification.
According to one aspect of the present invention, a stereomicroscope comprises an illumination unit for illuminating a specimen with light, a specimen setting board, and a fitting member for fitting an objective lens. The illumination unit, the specimen setting board and the fitting member are disposed in sequence on an optical axis. One of a predetermined a low-magnification objective lens and a higher-magnification objective lens than the low-magnification objective lens can be selected and fitted as the objective lens to the fitting member. The illumination unit includes a light source, a shield element for cutting off partially light beam emitted from the light source, first and second condenser lenses for converging the light beam passing the shield element on the specimen, and a mechanism for selecting one of the first and second condenser lenses and disposing the selected lens on the optical axis. The first condenser lens has an optical characteristic of setting a position conjugate to an entrance pupil of the low-magnification objective lens fitted to the fitting member in a position of the shield element or in the vicinity of the shield element, and the second condenser lens has an optical characteristic of setting a position conjugate to an entrance pupil of the high-magnification objective lens fitted to the fitting member in a position of the shield element or in the vicinity of the shield element.